This invention relates to a coating for a reflective surface, and, more particularly, to a coating which provides a uniform intensity in a polarized beam reflected from the reflective surface of a rotating polygon mirror over a wide range of incident angles.
Many conventional raster optical scanners utilize a multi-faceted rotating polygon mirror as the scanning element. A collimated beam of light, as for example from a laser, strikes the surface of the facets of the rotating polygon which causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon and scan a straight line. This reflected light can be utilized as a scanning beam to scan a document at the input end of an imaging system or can be used as an imaging beam to impinge upon a photosensitive medium in the output mode.
In raster optical scanners, it is essential that the intensity of the scanning beam be accurately controlled at the scan line for scanning or the imaging beam be accurately controlled at the recording member which typically comprises the photoreceptor in a xerographic system. Control over the beam's intensity is critical if the proper exposure level for the particular recording member is to be assured, and if variations in intensity across the scan line and from scan line to scan line, in the laser output power, and in transmittance, reflectance and throughput efficiency of the various optical components are to be compensated for. Some control techniques commonly used for this purpose are the addition of neutral density filters to the optical scanning system, making the entire laser tube assembly rotatable to permit the laser to be adjusted for polarization sensitive modulators, adjusting the rf drive power to the modulator for a recording member at the scan line by either varying the supply voltage or the amplitude of the image signals being input to the modulator, and adjusting the laser power supply.
However, the addition of neutral density filters and adjustment of the laser tube assembly are only capable of being implemented manually which limits their desirability. On the other hand, adjustments to the modulator drive power and to the laser power supply can be implemented in either manual or automatic fashion, the latter typically being in response to laser beam intensity, which renders these control techniques somewhat more desirable.
Notwithstanding, there are certain disadvantages with each of these control techniques. In particular, the addition of neutral density filters may induce flare light and beam aberrations. Permitting adjustment of the laser tube assembly can result in pointing errors in the laser beam and require subsequent realignment of the other optical components in the optical scanning system following each adjustment of the laser. And although adjustment of either modulator or laser power may be carried out automatically, adjusting the laser power supply is known to be impractical for gas type laser, which compromise the bulk of present day optical scanners, while adjusting the modulator drive power requires complex and expensive electronic circuits.
One proposed solution has been to use a twisted nematic liquid crystal or a magneto-optic cell to rotate the plane of a linearly polarized scanning beam transmitted through the crystal or cell to maximize beam intensity.
However, certain raster optical scanner applications require a uniform intensity, preferably at a maximum level, rather than just the maximum beam intensity. This uniform intensity is important for gray scale printing, for example. The more uniform the intensity of the output power of the imaging beam, the more uniform the print pattern across the printed page will be. This uniform intensity is also important for precise scanning of an input document.
Moreover, as the polygon mirror of the raster optical scanner rotates, the angle of the incident beam striking the reflective surface of a mirror facet will vary, as will, of course, the angle of reflection of the reflected beam from the mirror facet. Therefore, the intensity of the incident beam must be uniform over a wide range of angles as the beam strikes the rotating polygon mirror facet and is reflected across the scan line.
It is an object of this invention to provide a uniform intensity of a incident beam over a wide range of angles as the beam strikes the rotating polygon mirror facet and is reflected across the scan line.
It is another object of this invention to provide a simple means using just an incident beam and the rotating polygon mirror of an optical scanning system to provide a uniform maximum intensity for a reflected beam to be used in scanning or imaging.